Such a control device is known from U.S. Pat. No. 4,730,544. The two variable orifices are formed by an oblique blind bore formed in the radially inner surface of the outer sleeve and overlapping two grooves running axially in the radial outer surface of the inner sleeve.
Such a design allows a flow of hydraulic fluid through the control device even when the two sleeves are in a neutral position relative to each other. The device can be kept at approximately the same temperature as the hydraulic fluid to avoid thermal shock which could appear at a too high temperature difference between the control device and hydraulic oil entering the control device.
The device according to U.S. Pat. No. 4,730,544 operates satisfactorily when the standby pressure of the hydraulic oil is not too large, e.g. 8 bar only. However, in some case there is a demand for a higher standby pressure of the hydraulic fluid. In this case a noise appears which is in many cases not acceptable.